Effect of the plasma operating frequency on the figures of merit of an inductively coupled plasma time-of-flight mass spectrometer

Abstract

The analytical characteristics of a 27.12 MHz and a 40.68 MHz inductively coupled plasma time-of-flight mass spectrometer were evaluated under both hot and cool plasma conditions. The study was both direct and unambiguous because the same power supply, impedance-matching unit, load coil, and ICP torch were used at both operating frequencies. Although there are significant differences in the figures of merit between hot and cool plasma conditions, the performance at 27 MHz and 40 MHz was comparable. Under hot plasma conditions, measured ion energies and the plasma offset potential were nearly independent of the generator frequency. Higher ion energies and a greater plasma offset potential were apparent at 27 MHz under cold plasma conditions than at 40 MHz; however, this result may be an artifact of the difference in the optimal nebulizer flow rate rather than of frequency. Experimentally determined gas-kinetic and ionization temperatures did not demonstrate a significant frequency dependence.